Blend of a polyester and a polycarbonate

ABSTRACT

A polymeric blend comprising a polyester and a polycarbonate, wherein: the polyester contains recurring units having the structure &lt;IMAGE&gt;   and the polycarbonate contains recurring units having the structure   &lt;IMAGE&gt;

FIELD OF THE INVENTION

This invention relates to a blend of a polyester and a polycarbonateand, more particularly, to such a blend wherein the polyester and thepolycarbonate are very compatible; i.e., the polyester and thepolycarbonate can form a solid solution with each other.

BACKGROUND

Polycarbonates are well known and widely used engineering thermoplasticshaving utility for producing molded articles and protective plasticovercoats. They are known to exhibit good hardness, good impactresistance, and, in their amorphous higher molecular weight forms, goodtransparency. However, polycarbonates have drawbacks for someapplications, namely, poor resistance to stress cracking caused bycontact with solvents such as gasoline or other chemicals, and, in theirlower molecular weight forms, a tendency to crystallize and therebybecome hazy rather than transparent.

In contrast, some thermoplastic polyesters, while not having as highimpact resistance as polycarbonates, have excellent resistance to stresscracking caused by contact with solvents or other chemicals, and goodamorphousness and transparency over a wider range of molecular weights.

It would be desirable to be able to provide a polymeric composition, forexample by blending a polyester with a polycarbonate, that would providea combination of the beneficial properties of both the polyester and thepolycarbonate, e.g., good hardness, high impact resistance, hightransparency, good resistance to chemical-caused stress cracking, andrelatively reasonable cost. Unfortunately, these goals have beenthwarted in the past by the inherent incompatibilities of manypolyesters and polycarbonates. By "incompatibility", we mean theinability of the two polymers to form and/or maintain a solid solutionwith each other over a wide range of proportions and externalconditions. Incompatibility of a blend of two or more polymers isevidenced by the blend's having more than one glass transitiontemperature (Tg), which is a reliable indicator that the polymers in theblend exist in separate phases, each exhibiting properties differentfrom each other, rather than the single set of properties exhibited by ablend of compatible polymers that form a solid solution with each other,i.e., form a single phase in the blend. Inherent drawbacks of phaseseparations between the polymers in a blend include: inability toprovide a single Tg, poorer structural integrity, poorer resistance toimpact and other stress, poorer resistance to cracking caused bychemical attack, and poorer transparency due to light scattering, all ofwhich are contrary to the purposes intended to creating the blend.

Thus, a need exists for a blend of a polyester and a polycarbonate,wherein the blended polymers are compatible with each other over a widerange of proportions, and wherein the blend exhibits good amorphousnessand transparency, good impact resistance, a single Tg, better resistanceto chemically caused stress cracking than that of the polycarbonateitself, and reasonable cost.

SUMMARY OF THE INVENTION

The present invention satisfies the above-noted need by providing apolymeric blend comprising a polyester and a polycarbonate, wherein:

the polyester contains recurring units having the structure ##STR3## andthe polycarbonate contains recurring units having the structure ##STR4##

The polyester and polycarbonate in the blend of this invention are verycompatible with each other; they form a solid solution with each otherwhen combined in any proportions. The blend exhibits a single Tg, goodamorphousness and transparency over a wide range of molecular weights ofboth polymers in the blend, good impact resistance, and betterresistance to chemically caused stress cracking than that of thepolycarbonate itself. The blend can be obtained at reasonable cost, andhas utility for producing molded or cast articles and protectiveovercoats.

DESCRIPTION OF PREFERRED EMBODIMENTS

The polymers having recurring units of Structure (II) employed in theblends of this invention are well known "bisphenol Z" polycarbonates andcan be prepared by any of the methods well known in the art forsynthesizing such polycarbonates. Bisphenol Z polycarbonates havingweight average molecular weights within the range of from 10,000 to200,000 are useful in accordance with the invention.

The polymers having recurring units of Structure (I) employed in theblends of this invention are polyesters and can be prepared by methodsgenerally known to be useful for polyester syntheses, e.g., bycondensation of appropriate deceits (or their esters or salts) withappropriate diols. For example, an appropriate diacid salt is2,5-dimethylterephthaloyl chloride, can be prepared by condensation ofthionyl chloride with the diacid, 2,5-dimethylterephthalic acid, whichis readily commercially available, e.g., from the Buder Subsidiary ofthe Aldrich Chemical Co., USA or from Chem Service, Inc., West ChesterPa., USA. An appropriate diol is tetraemethylbisphenol A, which can beprepared by condensation of 2,6-dimethylphenol with acetone. Furtherdetails of preparations of the diacid salt, the diol, and the polyesterare presented in Preparations 1-3, below. Polyesters having recurringunits of Structure (I), that are useful in accordance with theinvention, have weight average molecular weights within the range offrom 10,000 to 200,000.

Blends in accordance with the invention contain the polyester and thepolycarbonate in any desired proportions, so long as each is present ingreater than zero amount. The particular proportions chosen will dependupon the particular balance of properties desired. However, whateverproportions are chosen, it has been unexpectedly found that thepolycarbonate having recurring units of Structure (II) is fullycompatible with the polyester having recurring units of Structure (I);i.e., the polyester and polycarbonate can be blended in any proportionsto form a solid solution having a single phase and a single Tg.

Blends of this invention can be formed by any means well known in theart for preparing a solid solution of different polymers. For example,the polymers can be dissolved and well-mixed in any solvent in whichboth polymers are fully soluble (e.g., dichloromethane) followed bydrying off the solvent (to produce the blend in bulk), or mixing thesolution with a liquid in which the polymers are not soluble (toprecipitate the blend in particulate form), or followed bysolvent-coating a layer of the solution onto a substrate and then dryingoff the solvent to form an overcoat film of the blend on the substrate,which can remain on the substrate as a permanent overcoat or be peeledoff the substrate to form a free-standing film. Alternatively, forexample, the polymers can be physically melt-blended in any suitabledevice (e.g., an extruder) at a temperature high enough to allow easyflow and thorough mixing of both polymers followed by cooling in bulk orchopped pellet form or melt-coating a layer or molding into a desiredshape and then cooling. The method of blending is not critical, so longas it enables thorough mixing and the formation of a solid solution.

The following preparations and examples are presented to furtherillustrate some blends in accordance with the invention and to comparetheir properties to those of blends or single polymers outside the scopeof the invention.

A polyester containing recurring units having Structure (I) wassynthesized as described in Preparations 1-3, below.

PREPARATION 1 2,5-dimethylterephthaloyl chloride

In a 2-liter 3-necked round-bottom flask, equipped with a stirrer,condenser, and nitrogen gas inlet, was placed 171 g (0.88 mol) of2,5-dimethylterephthalic acid, 500 g of thionyl chloride, and 5 ml ofdimethyl formamide. The mixture was heated to reflux under nitrogenuntil the solution became clear (about 18 hours). The excess thionylchloride was evaporated under reduced pressure. The residue was taken upin hot hexane which was then removed under reduced pressure. The residuewas recrystallized from hot hexane to obtain 156 g (77%) of shiny whitecrystals of the desired product, 2,5-dimethylterephthaloyl chloride.

PREPARATION 2 Tetramethylbisphenol A

In a 1-liter 3-necked round-bottom flask equipped with a condenser,stirrer and HCl gas inlet tube, were placed 244 g (2.0 mol) of2,6-dimethylphenol and 116 g (2.0 mol) of reagent grade acetone. HCl gaswas then bubbled into the reaction mixture for approximately 5 hours(i.e., until the mixture was saturated with HCl). The reaction mixturewas stirred at room temperature for 24 hours, and the solids werefiltered and washed twice with 1 liter of hexanes, followed by 1 literof distilled water, then again with hexanes. The crude product wasrecrystallized from 1.5 liters of 80% aqueous methanol, collected, anddried in a vacuum oven at 50° C. for 24 hours to give 185 g (65) of thedesired product as white crystals.

Melting point=164° C.

Elemental Analysis: calculated for C₁₉ H₂₄ O₂ : 80.2% C, 8.5% H; Found:80.2% C, 8.5% H.

PREPARATION 3 Poly(tetramethylbisphenol A 2,5-dimethylterephthalate),Structure (I)

To a stirred mixture of tetramethylbisphenol A (28.44 g, 0.10 mol) andtriethylamine (22.3 g, 0.22 mol) in methylene chloride (100 ml) at 10°C. was added a solution of 2,5-dimethylterephthaloyl chloride (23.4 g,0.10 mol) in methylene chloride (70 ml). After addition, the temperaturewas allowed to rise to room temperature, and the solution was stirredunder nitrogen for 4 hours, during which triethylamine hydrochlorideprecipitated in a gelatinous form and the solution became viscous. Thesolution was then filtered and washed with dilute hydrochloric acid, 2%(100 ml) followed by water (3×200 ml). The solution was then poured intomethanol with vigorous stirring, and a white fibrous polymer, thedesired product, precipitated. The isolated polymer was dried in avacuum oven at 40° C. for 24 hours.

Weight average molecular weight=27,100.

Number average molecular weight=12,300.

(Molecular weights were determined by gel permeation chromatographybased on polystyrene equivalents.)

Glass transition temperature (by differential scanning calorimetry)=170°C.

EXAMPLES 1-3

Blends in accordance with the invention were prepared by thoroughlydissolving and mixing in dichloromethane various proportions ofStructure (II) polycarbonate, (obtained commercially from the TorayChemical Co., Japan, and having a weight average molecular weight of135,000 and a number average molecular weight of 23,400) and thepolyester of Structure (I) (prepared in accordance with Preparations1-3, above), solvent casting the various solutions, and then drying offthe solvent to yield free-standing films of the blends.

Each of the blends of Examples 1-3 exhibited good amorphousness andtransparency, good impact resistance, better resistance to chemicallycaused stress cracking than that of the Structure II polycarbonateitself, and a single Tg (determined by differential scanningcalorimetry).

For purposes of comparison, control films outside the scope of theinvention were prepared as in Examples 1-3, except that Control A wascomposed of 100% of the Structure (II) polycarbonate, Control B wascomposed of 100% of the Structure (I) polyester, and Control C wascomposed of a 50:50 weight ratio blend of the Structure (II)polycarbonate and a polyester formed from2,2-bis(4-hydroxyphenyl)propane and terephthalic:isophthalic acids(55:45 molar ratio) (a polyester sold under the trademark, Ardel D-100,by Amoco, Inc., USA). The Control C film had a hazy visual appearance(i.e., poor transparency) and exhibited two separate Tg's (determined bydifferential scanning calorimetry), which indicates that the polymers inthe blend were incompatible and formed separate phases.

Relevant data is presented in Table I, below.

                  TABLE I                                                         ______________________________________                                                   Weight Ratio     Tg('s)                                            Example    (polyester:polycarbonate)                                                                      (°C.)                                      ______________________________________                                        Control A   0:100           179                                               1          25:75            183                                               2          50:50            181                                               3          75:25            178                                               Control B  100:0            170                                               Control C  50:50            181 and 192                                       ______________________________________                                    

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it should be appreciated thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. A polymeric blend comprising a polyester and apolycarbonate, wherein:the polyester contains recurring units having thestructure ##STR5## and the polycarbonate contains recurring units havingthe structure ##STR6##
 2. The polymeric blend of claim 1, wherein thepolyester has a weight average molecular weight of from 10,000 to200,000, and the polycarbonate has a weight average molecular weight offrom 10,000 to 200,000.
 3. The polymeric blend of claim 1, having only asingle glass transition temperature, that temperature being at least170° C.